1. Field of the Invention
The present invention relates to a material configuring a turbine rotor of a steam turbine into which high-temperature steam flows as a working fluid, and more particularly to an Ni-base alloy for a turbine rotor of a steam turbine excelling in high-temperature strength and the like, and a turbine rotor of a steam turbine made of the Ni-base alloy.
2. Description of the Related Art
For a thermal power plant including a steam turbine, a technology for suppression of the emission of carbon dioxide is being watched with interest in view of the global environmental protection, and needs for highly efficient power generation are increasing.
To increase the power generation efficiency of the steam turbine, it is effective to raise the turbine steam temperature to a high level, and the recent thermal power plant having the steam turbine has its steam temperature raised to 600° C. or higher. There is a tendency that the steam temperature will be increased to 650° C., and further to 700° C. in future.
A turbine rotor, in which moving blades rotated by high-temperature steam are implanted, has a high temperature by circulation of high-temperature steam and generates a high stress by rotating. Therefore, the turbine rotor is required to withstand a high temperature and a high stress, and a material configuring the turbine rotor is demanded to have excellent strength, ductility and toughness in a range of room temperature to a high temperature.
Particularly, if the steam temperature exceeds 700° C., a conventional iron-based material is poor in high-temperature strength, so that the application of the Ni-base alloy is considered in for example JP-A 7-150277(KOKAI).
The Ni-base alloy has been applied extensively as a material mainly for jet engines and gas turbines because it is excellent in high-temperature strength and corrosion resistance. As its typical examples, Inconel 617 alloy (manufactured by Special Metals Corporation) and Inconel 706 alloy (manufactured by Special Metals Corporation) have been used.
As a mechanism to enhance the high-temperature strength of the Ni-base alloy, Al and Ti are added to secure the high-temperature strength by precipitating a precipitated phase called as a gamma prime phase (Ni3(Al, Ti)) or a gamma double prime phase, or both of them within the mother phase material of the Ni-base alloy. For example, there is Inconel 706 alloy which secures high-temperature strength by precipitating both the gamma prime phase and the gamma double prime phase.
Meanwhile, the high-temperature strength of Inconel 617 alloy is secured by reinforcing (solid-solution strengthening) the mother phase of Ni group by adding Co and Mo.
As described above, it is being studied to apply the Ni-base alloy as a material for a turbine rotor of a steam turbine having a temperature exceeding 700° C., and it is also considered that its high-temperature strength can be improved further more. And, the high-temperature strength of the Ni-base alloy is demanded to be improved by compositional modification or the like while maintaining the forgeability and weldability of the Ni-base alloy.